Steel hollow-head screw

ABSTRACT

The invention relates to a hollow-head screw which is made by simple successive cold working operations using a steel comprising between 0.15 and 0.25% of carbon.

The present invention relates to the field of screws.

More precisely, the present invention relates to the field of steelscrews having socket heads.

The present invention applies particularly, but not exclusively, tomaking screws for securing wheels to motor vehicles.

Numerous steel screws have already been proposed.

Nevertheless, the person skilled in the art is aware of the difficultyof making a socket in the head of such a screw.

At present, most steel socket-head screws are made using methods thatinclude heat treatment.

The person skilled in the art is aware that without such heat treatment,the depth that can be reached by the socket has until now been verylimited, typically to a maximum of 0.4 to 0.5 times the diameter.

An object of the present invention is thus to propose novel meansenabling a socket-head screw to be made that presents a deep socket,while avoiding any heat treatment.

An object of the present invention, and thus a characteristic thereof,is to make a screw presenting a socket head in which the socket has adepth that is greater than 0.6 times, and advantageously greater than0.8 times, and preferably greater than 0.85 times the diameter of thesocket.

In the context of the present invention, this object is achieved by asocket-head screw made merely by successive cold working operationsusing a steel having a carbon content lying in the range 0.15% to 0.25%,preferably in the range 0.19% to 0.23%.

According to another advantageous characteristic of the presentinvention, the percentage of manganese lies in the range 1.00% to 1.50%,and preferably in the range 1.00% to 1.30%.

According to another advantageous characteristic of the presentinvention, the percentage of boron lies in the range 10 parts permillion (ppm) to 50 ppm, and preferably in the range 20 ppm to 50 ppm.

According to another advantageous characteristic of the presentinvention, the material constituting the screw includes microadditionsof titanium, preferably in the range 0.01% to 0.10%, and preferably inthe range 0.02% to 0.05%.

The Applicant has found that the correlation between titaniummicroadditions limiting the phenomenon of recrystallization, and thecontrolled cooling during rolling sequences, makes it possible to obtaina ferrito-pearlitic metallurgical structure that is very fine and veryuniform.

According to another advantageous characteristic of the presentinvention, the raw rolled wire used for making the screws presents thefollowing mechanical properties: minimum tensile strength (Rm)>580megapascals (MPa) and minimum yield point (Re)>340 MPa. The Applicanthas also found that such mechanical properties are particularly welladapted to forging screws without having recourse to conventional heattreatment operations: globular annealing before forging; andquenching+tempering after forging.

Also preferably, the wire presents ductility Z %>65%.

Other characteristics, objects, and advantages of the present inventionappear on reading the following detailed description with reference tothe accompanying drawing given as non-limiting examples, and in which:

FIG. 1 is a diagrammatic side view of a screw in accordance with thepresent invention;

FIG. 2 is an end view of the screw; and

FIG. 3 is a longitudinal section view of a screw constituting aparticular and non-limiting embodiment of the present invention.

Accompanying FIGS. 1 to 3 are diagrams showing a screw in accordancewith the present invention.

Nevertheless, the present invention is not limited in any way to theparticular dispositions shown in the accompanying figures, whetherconcerning the shape or the dimensions of the threaded portion 10, ofthe threads themselves, or of the head 20, and in particular of thesocket 22 formed therein.

Nevertheless, it should be observed that in the non-limiting embodimentshown in the accompanying figures, the head 20 possesses an outsidesurface 24 of hexagonal section, while the socket 22 possesses an insidesurface that is circularly cylindrical.

By way of non-limiting example, such a head may have a socket 22 with aninside diameter equal to 11.5 millimeters (mm), and a depth equal to 9.8mm.

Furthermore, and still in non-limiting manner, it should be observedthat in the particular and preferred embodiment shown in theaccompanying figures, the segment 24 of hexagonal section is connectedto the threaded portion 10 successively via a first cone 25 flaringtowards the threaded portion, by a second cone 26 converging towards thesame threaded portion, a smooth cylinder 27, and a groove 28.

In the context of the present invention, the material constituting thescrew preferably satisfies the following characteristics:

-   -   C content: 0.19% to 0.23%;    -   Si content: 0.30% to 0.40%;    -   Mn content: 1.00% to 1.30%;    -   P content: 0.025% max;    -   S content: 0.015% max;    -   Cr content: 0.14% to 0.18%;    -   Mo content: 0.05% max;    -   Cu content: 0.25% max;    -   B content: 0.0020% to 0.0050% max;    -   Ni content: 0.18% max;    -   Al content: 0.02% to 0.06%;    -   Ti content: 0.02% to 0.05%; and    -   N content: 0.012% max.

The screw is preferably made using rolled or drawn wire satisfying thefollowing mechanical characteristics:

-   -   Rm: 580 MPa to 650 MPa;    -   minimum Re: 340 MPa;    -   minimum elongation (A): 25%;    -   minimum Z: 65%.

After work hardening to a ratio of 50%, the screw presents the followingcharacteristics:

-   -   Re: 640 MPa to 900 MPa;    -   Rm: 800 MPa to 950 MPa;    -   minimum A: 12%;    -   minimum U-notch fracture toughness (KCU): 70 Joules per square        centimeter (J/cm²).

As mentioned above, the Applicant has found that this compositionenables screws to be made merely by cold working, without requiring anyheat treatment.

Naturally, the present invention is not limited to the particularembodiments described above, but extends to any variant within thespirit of the invention.

1. A socket-head screw made merely by successive cold working operationsusing a steel having a carbon content lying in the range 0.15% to 0.25%.2. A screw according to claim 1, characterized by the fact that thecarbon content lies in the range 0.19% to 0.23%.
 3. A screw according toclaim 1 or claim 2, characterized by the fact that it has a socket headin which the depth of the socket is greater than 0.6 times andpreferably greater than 0.8 times its diameter.
 4. A screw according toany one of claims 1 to 3, characterized by the fact that the manganesecontent lies in the range 1.00% to 1.50%, and preferably in the range1.00% to 1.30%.
 5. A screw according to any one of claims 1 to 4,characterized by the fact that the boron content lies in the range 10ppm to 50 ppm, and preferably in the range 20 ppm to 50 ppm.
 6. A screwaccording to any one of claims 1 to 5, characterized by the fact thatits constituent material includes microadditions of titanium.
 7. A screwaccording to any one of claims 1 to 6, characterized by the fact that itincludes 0.01% to 0.10% titanium, and preferably 0.02% to 0.05%.
 8. Ascrew according to any one of claims 1 to 7, characterized by the factthat the silicon content lies in the range 0.30% to 0.40%.
 9. A screwaccording to any one of claims 1 to 8, characterized by the fact thatthe chromium content lies in the range 0.14% to 0.18%.
 10. A screwaccording to any one of claims 1 to 9, characterized by the fact thatthe sulfur content is 0.015% max.
 11. A screw according to any one ofclaims 1 to 10, characterized by the fact that it is made using a wirepresenting the following mechanical properties: Rm>580 MPa and Re>340MPa.
 12. A screw according to any one of claims 1 to 11, characterizedby the fact that it is made using a wire presenting ductility Z %>65%.13. A screw according to any one of claims 1 to 12, characterized by thefact that it is made using the following materials: C content: 0.19% to0.23%; Si content: 0.30% to 0.40%; Mn content: 1.00% to 1.30%; Pcontent: 0.025% max; S content: 0.015% max; Cr content: 0.14% to 0.18%;Mo content: 0.05% max; Cu content: 0.25% max; B content: 0.0020% to0.0050% max; Ni content: 0.18% max; Al content: 0.02% to 0.06%; Ticontent: 0.02% to 0.05%; and N content: 0.012% max.
 14. The use of ascrew in accordance with any one of claims 1 to 13 in making screws forsecuring wheels to motor vehicles.